Industrial sewing machines have long been used for sewing together relatively thick materials such as mattress cover$ and upholstery. Sewing machines used for sewing such thick materials must be adapted to provide adequate vertical clearance for the material to fit through the throat of the machine. The throat plate, the plate upon which the material rests as it is sewed, defines the bottom plane of the space in which the material must fit in the throat of the machine. Accordingly, the thickness of the material which can be sewn by the machine is dictated by the clearance between the throat plate and the lowest of the upper travel limit of the 1) needle, 2) presser foot and 3) upper walking foot. Although the needle, presser foot and upper walking foot must provide an unusually large clearance when at their upper travel limit, the needle must still be able to plunge below the throat plate to place a stitch in the material. Accordingly, the needle's throw (i.e., the distance traveled by the needle from the top of its motion to the bottom) on such a machine must also be unusually long. If the machine is to be able to be used with thin as well as thick materials, the same is true of the throw of the presser foot and the upper walking foot. The presser foot and upper walking foot must be able to get very close to the throat plate in order to contact very thin materials but also must be able to provide larger clearance for very thick materials.
Typically, the lower travel limit of the presser foot in a sewing machine is limited by contact of the presser foot with the throat plate. However, this manner of limiting motion is undesirable because the impact of the presser foot with the throat plate causes unnecessary wear and tear on the components of the device. It is also desirable to be able to adjust the downward force of the presser foot on the material in order to accommodate for materials of different thickness.
In sewing machines which comprise upper walking feet, the upper walking foot is typically positioned directly over the lower walking foot so that the material is "clamped" between the upper and lower walking feet. U.S. Pat. No. 4,449,464 discloses a sewing machine including upper and lower walking feet. Some sewing machines also include a secondary (or rear) lower walking foot spaced rearwardly of the upper walking foot and primary lower walking foot. Reference is made to U.S. Pat. Nos. 4,166,422, 3,995,571 and 3,530,809 as examples of sewing machines comprising mating upper and lower walking feet and an additional secondary lower walking foot spaced rearwardly away from the other walking feet. In certain sewing situations, it is desirable for the various walking feet to move at different speeds relative to one another. This is typically useful where one wishes for the material to bunch as it is sewed. For instance, when bunching of the material is desired, the forward lower walking foot should move at a speed slower than the rearward lower walking foot. Forward is defined herein as the direction in which the material is advanced as it is sewn. In fact, in certain applications, it is desirable for the upper walking foot to travel at a different speed than the primary lower walking foot (which it is directly above), thus causing the upper layer of material to travel at a different speed than the lower, causing only one of the layers to bunch as it is sewed.
In overcast stitching, thread is wrapped once around the edge of the material for each stitch. In machines adapted for performing overcast stitching, two strands of thread, an upper strand which is looped through an eye in the tip of the needle and a lower strand which is looped through a looper below the throat plate, are intertwined to form the overcast stitches. The stitch will not be described herein as it is conventional in the art and not material to the present invention except insofar as improvements have been made to certain mechanical components of an overcast stitching sewing machine. Accordingly, the following discussion of overcast stitching is not intended to be complete but simply describes the necessary machine components for accomplishing overcast stitching.
FIGS. 1A 1B and 1C illustrate the movement of the needle, looper and spreader of an overcast stitching sewing machine through one stitch cycle. The strands of thread are not shown for ease of illustration. When the tip of the needle 70 is plunged below the throat plate 11, as shown in FIG. 1A, a looper 183, rotating about axis 187 in the direction of arrow 65, passes close by the needle tip and traps the upper thread from needle 70 underneath arm 183a. Looper 183 transports the trapped thread horizontally beyond the edge of the material 77 as the needle continues its stroke and begins moving upwardly, as illustrated in FIG. IB. When the looper reaches beyond the edge of the material (FIG. 1B), spreader 180, moving upwardly, grabs the lower thread from looper 183 by trapping it in fork 182 and transports it around the edge of the material bringing it up and over the material and positioning it adjacent the tip of needle 70 as the needle reaches the top of its throw and begins its downward stroke again (FIG. 1C). The needle removes the thread from the spreader by trapping it between the needle tip and the upper thread strand which passes through the eye 67 in the needle tip. At the moment when the thread is trapped by needle 70, spreader 180 reaches the top of its stroke and reverses direction, moving downwardly and to the right, releasing the thread from fork 182. The needle then plunges through the material transporting the thread grabbed from the spreader through the material causing a stitch to be formed over the edge of the material returning to the position shown in FIG. 1A. The apparatus then repeats the cycle to make the next stitch.
Commonly, an overcast stitching machine is also provided with a knife system comprising upper and lower knives which cut the material just before it is advanced through the sewing area so as to form the edge close to the needle and parallel to the stitching. The knife assembly is rearwardly of the sewing area and typically comprises a stationary lower knife and a reciprocating upper knife which moves up and down to meet with the lower knife during the lower portion of its travel to act as a scissor to cut the material before it is advanced into the sewing area.
Accordingly, it is an object of the present invention to provide an improved sewing machine.
It is a further object of the present invention to provide a sewing machine with a very large vertical clearance between the throat plate and the upper travel limits of the needle, presser foot and upper walking foot.
It is yet another object of the present invention to provide a sewing machine with variable speed walking feet.
It is yet another object of the present invention to provide a sewing machine in which the lower travel limit of the presser foot is adjustable relative to the throat plate.
It is a further object of the present invention to provide a sewing machine in which the downward force of the presser foot can be set to any desired force.
It is another object of the present invention to modify existing sewing machines to improve their utility.
It is yet one more object of the present invention to provide an overcast stitch sewing machine having high clearance for accepting thick materials and having an increased throw and upper travel limit of the spreader.